The present invention relates to a liquid crystal display apparatus, particularly, to an active matrix liquid crystal display apparatus and IC (Integrated Circuit) cards.
Currently, an active matrix liquid crystal display apparatus using active elements, represented by TFT (Thin Film Transistor) devices, have been widely used as monitors for personal computers and work stations, in view of their high image quality, which is comparable to a CRT (Cathode Ray Tube), their low power consumption, which is less than a CRT, and saving space. However, an active matrix liquid crystal display apparatus is expensive in comparison with a CRT, and the lowering of its price is required in order to provide for wider use of this apparatus.
As one of the methods for lowering its price, application of an organic thin film transistor (organic TFT), which can be manufactured readily, to the active element has been proposed. However, both a plasma Chemical Vapor Deposition (CVD) apparatus for manufacturing insulating layers and semiconductor layers of the current amorphous silicone TFT and a spattering apparatus for manufacturing electrodes are very expensive. Furthermore, the CVD method requires a high temperature in the range of 230-350xc2x0 C. for forming films, and the throughput of the apparatus is low, because the apparatus requires frequent maintenance, such as cleaning and the like. On the other hand, an application apparatus and a vapor deposition apparatus for manufacturing an organic TFT is cheap in comparison with the cost of a CVD apparatus and a spattering apparatus. The temperature for forming films using the above apparatus is lower, and their maintenance is easier in comparison with the CVD apparatus and the spattering apparatus. Therefore, a significant decrease in production cost can be realized by using an organic TFT for a liquid crystal display apparatus.
Generally, an organic TFT is composed of a glass substrate, gate electrodes, a gate insulating film, source electrodes, drain electrodes, and an organic semiconductor film. The amount of charge at a boundary between the gate insulating film and the organic semiconductor can be altered from excessive to deficient, or vice versa, by varying the voltage applied to the gate electrode. Switching is performed by varying the value of drain current flowing through the source electrode/the organic semiconductor/the drain electrode.
A method for manufacturing an organic TFT using a sexithiophene oligomer vapor deposition film as the organic semiconductor film was disclosed in JP-A-8-228035 (1996). In (Y-Y, Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson; IEEE Transactions on Electron Devices, Vol. 44, No. 8, p 1325-1331 (1997)), a method of manufacturing a high performance organic TFT using pentacene vapor deposition film as the organic semiconductor film was disclosed. JP-A-8-191162 (1996) disclosed a method for manufacturing an organic TFT using organic materials for the semiconductor film, source electrodes, drain electrodes, and gate electrodes, wherein the gate insulating film is made of an insulating polymer having cyano-groups.
In accordance with JP-A-8-228035 (1996) and JPA-10-125924 (1998), the organic semiconductor films were formed by vapor deposition methods, but pattern formation of the semiconductor was not described. For instance, when the pattern is formed using a metallic mask made of molybdenum, the minimum size of the pattern becomes approximately 100 xcexcm, which is larger than a pixel size (10xc3x9730 xcexcm2) in the current liquid crystal display apparatus. When conventional lithography (photolithography) is used, deterioration by carrier injection into the semiconductor layer and the like in an annealing process for removing a polar solvent or a solvent used for resist materials becomes a concern. A two terminal electron element having an organic semiconductor layer, wherein a pattern was formed between two parallel electrodes on a substrate, was disclosed in JP-A-2-239663 (1990). In accordance with JP-A-2239663 (1990), a patterned insulating film, wherein electrode portions were removed, was formed on a lower electrode, and an organic semiconductor film having the same size as the lower electrode was formed using the lower electrode. However, the material was restricted to an organic material which would be an electron donor, such as tetracyanoquinonedimethane and the like, and other materials can not be used. Furthermore, the technique can not be applied to three terminal elements, such as a TFT and the like.
With respect to a conventional organic TFT, the organic semiconductor film can not be manufactured finely, and the area of the organic semiconductor film is larger than the gate electrodes. Therefore, an off-current caused by wraparound is increased. On account of the large area of the organic semiconductor film, the organic semiconductor film can not be covered by a light shielding layer, and off current with carriers generated by photo excitation is increased. As a result, an on/off ratio indicating the performance of the switching element is decreased. If the on/off ratio is decreased, for instance, when the device is used in a liquid crystal display active element, the applied voltage to the liquid crystal is decreased because the current flows even in an off condition, and the holding characteristics are deteriorated.
In accordance with a decrease in the off-current, the amount of gate voltage variation (S-value) necessary for increasing the drain current by one order, which is an index indicating the speed of the switching action, is increased, and the TFT characteristics are deteriorated.
If the above organic TFT is used for the active elements in the liquid crystal display apparatus, writing into a liquid crystal pixel may be produced by an adjacent signal line and lowering of the contrast results, because a TFT is composed of an organic semiconductor film between the drain electrode and the adjacent signal line.
One of the objects of the present invention is to provide a method of forming patterns, which makes it possible to avoid deterioration the TFT characteristics of the organic TFT element and a lowering of the contrast in the liquid crystal display apparatus due to the influence of an adjacent signal line.
In an organic thin film transistor comprising a substrate, gate electrodes, gate insulating layers, source electrodes, drain electrodes, and an organic semiconductor layer, the above object can be realized by making channel regions of the organic semiconductor film formed as patterns of the same size as the gate electrodes via the insulating layer which is formed as a pattern between the gate insulating layer and the semiconductor layer.
In accordance with the present invention, a photosensitive insulating film can be used instead of the insulating layer.
The present invention is characterized by use of such an organic thin film transistor as an active element in an active matrix liquid crystal display apparatus.
The organic TFT referred to here comprises conductive gate electrodes, gate insulating layers, source electrodes and drain electrodes which are arranged relative to each other horizontally with an interval between them, and an organic semiconductor layer. The organic TFT operates with either of an accumulating condition and a vacant condition depending on the polarity of the voltage applied to the gate electrodes.
The gate electrode relating to the present invention is arranged in the region between the source electrode and the drain electrode, and above or beneath the region having a longitudinal direction of the source/drain electrode as one side. The size of the gate electrode is desirably in the range from 1.1 times to 1.2 times of respective sides of the above regions in consideration of the position adjustment. As the material of the electrode, an organic material, such as polyaniline, polythiophene, and the like, or a conductive ink, which can be readily formed in the shape of the electrode by a coating method, is desirable. It is also desirable to use a metallic material, such as gold, platinum, chromium, palladium, aluminum, indium, molybdenum, nickel, and others, which can be formed in the shape of the electrode by conventional photolithography; an alloy made of the above metals; and an inorganic material, such as polysilicone, amorphous silicon, tin oxide, indiumoxide, indium-tin-oxide (ITO), and others. Naturally, the invention is not restricted to the above materials, and at least two kinds of the above materials can be used concurrently.
As the material for the gate insulating film of the present invention, an organic material, such as polychloropyrene, polyethylene terephthalate, polyoxymethylene, polyvinyl chloride, polyvinylidene fluoride, cyanoethyl-pullulan, polymethylmethacrylate, polysulfon, polycarbonate, polyimide, and others, which can be formed in the shape of the electrode by a coating method in the same manner as the material for the gate electrode, is desirable. Furthermore, an inorganic material, such as SiO2, SiNX, Al2O3, and others, which can be formed in the shape of the electrode by conventional photolithography, is desirable. Naturally, the invention is not restricted to the above materials, and at least two kinds of the above materials can be used concurrently.
As the material for the source electrode and the drain electrode of the present invention, a metal having a large work function is desirable in order to make ohmic contact with the semiconductor layer, because most of the organic semiconductor is a P-type semiconductor, wherein carriers for transporting charges are holes. Practically, gold and platinum are desirable, but the invention is not restricted to these materials. In a case when a dopant is doped onto the surface of the semiconductor layer with a high density, an energy barrier between the metal and the semiconductor is reduced and the carriers come to be able to tunnel it. Therefore the metallic materials described previously as the materials for the gate electrode can be used.
The patterned insulating film of the present invention is composed in a manner such that the patterned insulating film is formed between the gate insulating film and the organic semiconductor layer, and regions of the insulating film above or beneath the gate electrode are removed. The area of the region of the insulating film to be removed desirably has the same size as the size of the gate electrode. The patterned insulating film has a function as a mask pattern when forming the semiconductor film. That is, the semiconductor layer can be formed so as to contact the gate insulating film only in the regions, which operate as channel regions, by accumulating the semiconductor film onto the patterned insulating film after forming the patterned insulating film. The semiconductor film is formed via the patterned insulating film (together with the source electrode and the drain electrode in the regions where these electrodes are formed), in all the regions, except the regions where the insulating film is removed above or beneath the gate electrodes. Accordingly, the semiconductor film can be formed precisely in the channel regions.
The photosensitive insulating film of the present invention has concurrently a property to form a photo-pattern itself. Therefore, no resist material is required, and its manufacturing process can be shortened. As the material for the patterned insulating film, an insulating material different from the material for the gate insulating film must be used, in order to perform a selective etching operation.
Practical examples of the materials for the insulating film are inorganic materials such as SiO2, SiNX, Al2O3, and others, and organic materials such as polychloroprene, polyethylene terephthalate, polyoxymethylene, polyvinyl chloride, polyvinylidene fluoride, cyanoethyl-pullulan, polymethylmethacrylate, polysulfon, polycarbonate, polyimide, and others, but the insulating film is not restricted to the above materials.
As the materials for the organic semiconductor of the present invention, aromatic compounds having a n-electron conjugated group, aliphatic compounds, organic pigments, organic silicone compounds and others are desirable. Practically, pentacene, tetracene, thiophene oligomaer derivatives, phenylene derivatives, phthalocyanine compounds, polyacetylene derivatives, polythiophene derivatives, cyanine pigments, and others can be used as the materials for the organic semiconductor, but the invention is not restricted to the above materials.
The method for manufacturing the organic TFT of the present invention is characterized in comprising the steps of: forming gate electrodes, a gate insulating layer, source electrodes, and drain electrodes on a glass substrate; forming an insulating film thereon; removing the insulating film on the gate electrodes; and forming a semiconductor film thereon. As the method for manufacturing the organic TFT of the present invention, a plasma CVD method is used for the inorganic insulating film and others, and a spattering method is used for the metallic film, tin oxide, indium oxide, ITO and others. For forming a pattern, conventional photolithography, and a dry etching method or wet etching method, are used. An example of the manufacturing methods is disclosed in, S. Matsumoto: xe2x80x9cLiquid crystal display technology-active matrix LCDxe2x80x9d chapter 2, Sangyo tosyo (1996). As a method for manufacturing a thin film using conductive organic materials, conductive ink, insulating organic materials, and semiconductor organic materials as a raw material, a spin coating method, a casting method, a dipping method, a vacuum deposition method, and others can be used.
In accordance with the active matrix liquid crystal display apparatus referred to here, the active matrix element is attached to every pixel composing the display portion, and an voltage is applied to the liquid crystal via the active matrix element. Accordingly, its driving method is as follows.
An active element, such as TFT and others, is arranged respectively at every crossing point of a nxc3x97m matrix of lines with n rows of scanning lines and m columns of signal lines; the gate electrode of the TFT is connected to the scanning line, the drain electrode is connected to the signal line, and the source electrode is connected to the pixel electrode. Address signals are supplied to the scanning line, display signals are supplied to the signal line, and the liquid crystal on the pixel electrode is operated via a TFT switch, which is controlled by the address signals overlapped with on/off signals. By using an organic TFT as the switching element, the manufacturing process is simplified, and the price can be lowered.
Hitherto, the explanation has been given concerning use of an organic TFT, but the TFT structure and the manufacturing method of the present invention can be applied to a TFT having a semiconductor layer made of materials other than organic materials.